The broad long-term objective of this research is to increase understanding of the biological basis of male sexual motivation. This objective will be accomplished through studies of the correlates of individual differences in sexual motivation in rats and men. The specific aims of the research are to test the following hypotheses in two related studies: 1) Individual differences in male sexual motivation are related to the rewarding affective properties of testosterone, as measured by a conditioned place preference; 2) Individual differences in male sexual motivation are related to differences in pain sensitivity in humans as in rats. In animals, a within subject design will be used to measure relation between the rewarding affective properties of intra-accumbens injections of testosterone and individual differences in sexual behavior. Behavioral measures include the conditioned place preference paradigm where the rewarding properties of drug are inferred from a tendency of animals to approach previously neutral stimuli, and behavioral response to receptive females. The order of behavioral tested will be counter balanced across animals. The design of the study will permit an examination of the association between behavioral measures, as well as the possible interaction between sexual experience and affective properties of testosterone. In humans, a within subject design will be used to measure relations between pain sensitivity and individual differences in sexual behavior, variables that are known to be associated in male rats. The design of the study will permit an examination of the association between behavioral measures (sexual behavior, pain response), as well as the possible interactions between sexual experience and response to painful stimuli. The results of the research may provide information about the biological basis of individual differences in sexual behavior and may bear on the treatment of individuals with sexual dysfunctions.